Generally, in semiconductor devices such as the semiconductor laser devices and RF devices that are used in optical communications and the like, the problem of how to efficiently remove the heat generated by the devices is very important in order to prevent malfunctions or the like. In recent years, advances in semiconductor device technologies have led to higher outputs, higher speeds and higher integration of devices, placing ever stricter demands on their heat dissipation. For this reason, higher thermal conductivity is also generally required in heat-dissipating components such as heat sinks, and copper (Cu), which has a high thermal conductivity of 390 W/mK, is used.
On the other hand, individual semiconductor devices are becoming larger in size with the increased outputs, and the problem of mismatches in the thermal expansion between semiconductor devices and the heat sinks used for heat dissipation has gained prominence. In order to solve this problem, the development of a heat sink material that has the property of high thermal conductivity while also having a coefficient of thermal expansion that matches with that of semiconductor devices has been sought. As such materials, composites of metals and ceramics, for example, composites of aluminum (Al) and silicon carbide (SiC), have been proposed (Patent Document 1).
However, the thermal conductivity of an Al—SiC composite cannot be lowered to 300 W/mK or less no matter how the conditions are optimized, so the development of a heat sink material having even higher thermal conductivity, greater than the thermal conductivity of copper, has been sought. As such a material, a metal-diamond composite that combines the high thermal conductivity possessed by diamonds with the high coefficient of thermal expansion possessed by metals, and thus has high thermal conductivity and a coefficient of thermal expansion close to that of semiconductor device materials, has been proposed (Patent Document 2).
Additionally, in Patent Document 3, a β-type SiC layer is formed on the surfaces of diamond grains, thereby suppressing the generation of metal carbides having low thermal conductivity that are formed at the time of compositing, and improving the wettability with molten metals, thereby improving the thermal conductivity of the resulting metal-diamond composite.
Furthermore, since diamond is an extremely hard material, metal-diamond composites that are obtained by compositing diamond with metals are similarly very hard, difficult-to-work materials. For this reason, metal-diamond composites are mostly unworkable using normal diamond machine tools, and when using metal-diamond composites as heat sinks, which are small and have a variety of shapes, the issue of how to machine them at a low cost arises. In response to this issue, machining methods such as laser machining, waterjet machining, and furthermore, since metal-ceramic composites conduct electricity, electrical discharge machining, have been considered.
In heat-dissipating components for use in semiconductor devices, a metal layer must be added to the surface of the heat-dissipating component by plating or the like, in order to join them with devices. In the case of normal semiconductor devices, joining is mainly achieved by soldering, and since the joining temperature is 300° C. or less, a metal layer is provided on the surface by plating with an Ni—P alloy or the like.
However, in the mode of use as a heat sink material, in order to efficiently dissipate heat generated by a semiconductor device, the heat sink is usually arranged to be in contact with the semiconductor device by being joined by a brazing material or the like. For this reason, a multi-layer plating or the like, having gold plating applied to the joining surface, is used. Furthermore, in this type of application, as the joining temperature rises and the temperature load at the time of actual use increases, conventional alloy plating such as that using Ni—P alloys has the problem that swelling may occur due to the thermal expansion of the heat sink material and the plated film.
Furthermore, when a heat sink is joined to a semiconductor device by a brazing material or the like, the surface precision of the joint interface affects the heat-dissipating ability, and is therefore important. In the case of a conventional metal-diamond composite, diamond grains are exposed on the joint surface, so the surface roughness of the joint surface is high, and as a result, the thermal resistance of the contact interface unfavorably increases. For this reason, as a property sought in heat sink materials, there is the issue of how to lower the surface roughness of the surface.
Additionally, it is necessary to efficiently dissipate generated heat in order to raise the performance of semiconductor devices, and further high thermal conductivity materials are sought as heat sink materials.    Patent Document 1: JP H9-157773 A    Patent Document 2: JP 2000-303126 A    Patent Document 3: JP 2007-518875 A